Hybrid tensioning of riser string operating with energy storage device

ABSTRACT

An enhanced riser control system may employ electrical tensioners coupled to a drilling riser by wires. The electrical tensioners may provide quick response to a tension controller to handle positioning of the drilling riser. The electrical tensioners of the enhanced riser control system may be combined with hydro-pneumatic tensioners in a riser hybrid tensioning system. A controller within the enhanced riser control system may be configured to distribute tension to electrical tensioners and to control electrical tensioners to adjust the length of the first and second wires. Energy from an electrical tensioner may be transferred to an energy storage system or to power dissipaters for dissipating the energy generated by the electrical tensioner. The energy transferred from an electrical tensioner may be energy that has been generated by the electrical tensioner.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/963,019, filed Apr. 25, 2018, entitled “Hybrid Tensioning of RiserString Operating with Energy Storage Device,” which is a continuation ofU.S. patent application Ser. No. 15/072,206 filed on Mar. 16, 2016,entitled “Hybrid Tensioning of Riser String,” now U.S. Pat. No.9,963,944, which is a continuation of U.S. patent application Ser. No.13/715,412 filed on Dec. 14, 2012, entitled “Hybrid Tensioning of RiserString,” now U.S. Pat. No. 9,617,803, which claims the benefit of U.S.Provisional Application No. 61/725,411 entitled “Riser Hybrid TensioningSystem,” filed Nov. 12, 2012, and U.S. Provisional Application No.61/579,353 entitled “Enhanced Riser Control System,” filed Dec. 22,2011, the disclosure of each of which is hereby incorporated byreference.

TECHNICAL FIELD

This disclosure is related to riser control systems. More specifically,this disclosure is related to a riser tensioning control system havingelectrical tensioners.

BACKGROUND

Safety and performance are important considerations in a drilling riser.With trends over the past decades to exploit resources in deeper watersand harsher environments, ensuring the safety and performance ofdrilling risers has become a challenging task.

A riser tensioning system aims to compensate for relative motionsbetween a floating drilling rig and the seabed, which are joined by arigid riser string. In conventional systems, the most widely used risertensioning system is a hydro-pneumatic riser tensioning systemconsisting of hydro-pneumatic cylinders, air/oil accumulators, and airpressure vessels. However, there are short-comings in hydro-pneumatictensioning systems.

First, the response time for a hydro-pneumatic tensioning system is tooslow for certain situations. The relatively slow operation of pneumaticsystems results in a long control response time, which is the timebetween issuing a command and force being applied by the tension system.In certain situations, such as during an emergency riser disconnect, thetension changing response may be too slow. The slow, large over-pullingforce may accelerate free riser pipes outward, allowing them to jumpout, and consequently damage the drilling rig floor and riser pipes.

Second, increasing longitudinal over-pull tension, the conventionalmethod in hydro-pneumatic tensioning systems used to suppressdestructive vortex-induced vibration (VIV), causes stress on thesupporting equipment, increases wear and tear on the tensioning system,and increases riser pipe fatigue. Furthermore, increasing longitudinalover-pull tension raises safety concerns in situations where a pair ofhydro-pneumatic tensioners are receiving maintenance while the drillingrig is experiencing high wave conditions.

Third, a hydro-pneumatic tensioning system is a relatively complex andcostly system that requires a significant amount of maintenance and isat risk for hydraulic fluid leakage. A hydro-pneumatic tensioning systemincludes a hydro-pneumatic cylinder rod and a seal that are exposed tobending due to factors such as vortex-induced vibration (VIV) or unequaland non-linear loading caused by vessel roll and pitch. These factorsmay cause high failure risk and may require a high maintenance cost toavoid hydraulic fluid leakage and risks of environmental pollution.Furthermore, the complex hydro-pneumatic system includes a significantvolume of air accumulators and reservoirs that consume useful floorspace on a drilling rig.

SUMMARY

An enhanced riser tensioning system having an electrical tensioner mayprovide additional stability and performance over conventional risertensioning systems having only hydro-pneumatic tensioners. The systemmay enhance the overall safety and reliability of a deepwater risersystem. Electric tensioners have quicker response times thanhydro-pneumatic tensioners. With quicker response times, electrictensioners may apply variable tensions to provide more accurate heavecompensation control, safer anti-recoil control and reducing the fatiguedamage by vortex-induced vibration (VIV) on riser string. This riserhybrid tensioning system also brings new functionalities for simplifyingthe riser operation process, such as (1) a new riser position controloperation mode, (2) a new functionality of vessel motion stabilizer and(3) a new functionality of moving riser string between dual drillingstations

According to one embodiment, an apparatus includes a first and secondelectrical tensioner mechanically coupled to a drilling riser via afirst and a second wire of a plurality of wires and electrically coupledto a direct current (DC) power distribution bus. The apparatus may alsoinclude an energy storage system and a power dissipater, both of whichare also coupled to the DC power distribution bus. The apparatus mayfurther include a hydro-pneumatic tensioner mechanically coupled to thedrilling riser via a third wire of the plurality of wires. Further, theapparatus may include a controller configured to measure the tension andspeed delivered by both the electrical and hydro-pneumatic tensioner.The controller may also be configured to determine the tension for thefirst and second electrical tensioners based, in part, on the riser loadand the measured tension of the hydro-pneumatic tensioner. Thecontroller may be configured to distribute tension to the first andsecond electrical tensioners, and to control the first and secondelectrical tensioners to adjust the length of the first and secondwires.

The electrical tensioner within the apparatus may include a motorconfigured to act as a motor or a generator and an energy inverter. Theenergy inverter may be coupled to the motor and also to the DC powerdistribution bus. The electrical tensioner may further include a gearbox coupled to the motor and include a winch. The winch may be coupledto the gearbox and may be coupled to the drilling riser via the drillingriser wire. The energy inverter within the electrical tensioner mayinvert AC energy to DC energy or DC energy to AC energy. The controllermay be further configured to regulate the torque and power flow in aplurality of energy inverters.

Energy management may be improved on a vessel through the use of energystorage system. For example, energy may be stored in the storage systemwhen the electric tensioner operates as a generator to regenerate energyin the half wave motion of the vessel; and vice versa.

A method for controlling a tension of a riser tensioning system includesmeasuring a tension delivered by a tensioner. The method may alsoinclude determining a tension for a plurality of electrical tensionersbased, in part, on the measured tension. The method may further includedistributing the determined tension to the plurality of electricaltensioners. The method may also include controlling the plurality ofelectrical tensioners based, in part, on the determined tension. Themethod for controlling a tension of a riser tensioning system thatincludes distributing the determined tension to the plurality ofelectrical tensioners may be useful in stabilizing a riser in a drillingvessel.

In an embodiment, the delivered tension that is measured may be thetension of a hydro-pneumatic tensioner or an electrical tensioner. Insuch an embodiment, the tensioning system may be a riser hybridtensioning system, which is a riser tensioning system that integrates anelectrical tensioning system with hydro-pneumatic tensioners.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter which form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present disclosure. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the disclosure as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe disclosure, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed system and methods,reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings.

FIG. 1A is a block diagram illustrating a top view of a riser electricaltensioning system according to one embodiment of the disclosure.

FIG. 1B is a block diagram illustrating a top view of a riser hybridtensioning system according to one embodiment of the disclosure.

FIG. 2A is block diagram illustrating a riser tensioning systemaccording to one embodiment of the disclosure.

FIG. 2B is a block diagram illustrating a controller for the risertensioning system according to one embodiment of the disclosure.

FIG. 3A is a flow chart illustrating a method for controlling thetension of a riser tensioning system according to one embodiment of thedisclosure.

FIG. 3B is a flow chart illustrating a method for controlling energytransfer within a riser tensioning system according to one embodiment ofthe disclosure.

FIG. 4A is a graph illustrating a relationship between vessel velocityand riser tension according to one embodiment of the disclosure.

FIG. 4B is a graph illustrating a relationship between vessel velocityand riser tension according to one embodiment of the disclosure.

FIG. 4C is a graph illustrating tension applied by electric andhydro-pneumatic tensioners in a riser hybrid tensioning system accordingto one embodiment of the disclosure.

FIG. 5 is a block diagram illustrating routing of energy within a riserhybrid tensioning system according to one embodiment of the disclosure.

FIG. 6 is a block diagram illustrating a control scheme for energystorage devices according to one embodiment of the disclosure.

FIG. 7A is a block diagram illustrating a side and top view of adual-activity vessel having electric tensioners when a riser string ismoving from a first drilling station to the second station according toone embodiment of the disclosure.

FIG. 7B is a block diagram illustrating a side and bottom view of adual-activity vessel having electric tensioners when a riser string ismoving from a second drilling station to the first station according toone embodiment of the disclosure.

DETAILED DESCRIPTION

The safety and performance of a deepwater riser tensioning system may beimproved by using electrical components to control a tension of a riser.A riser hybrid tensioning system may integrate a riser electricaltensioning system with existing hydro-pneumatic tensioners to improvesafety and functionality over conventional riser tensioning systems. Ariser tensioning system may also include only electric tensioners.Electrical components, such as an electrical machine, can provide acontrol response in the range of milliseconds, which is a nearlyinstantaneous control response. Use of electrical components allowsquick response that improves safety and functionality by allowing thetensioning system to respond to different conditions faster. Moreover,additional functionality of a riser hybrid tensioning system may provideenhanced modes of operation to solve numerous problems encountered ondeepwater riser tensioning systems.

FIG. 1A is a block diagram illustrating a top view of a riser electricaltensioning system 150 according to one embodiment of the disclosure. Ariser 130 may be coupled to the electrical tensioners 110-117 by ropes.Although FIG. 1A depicts the electrical riser tensioning system 150 witheight electrical tensioners 110-117, the electrical riser tensioningsystem 150 is not limited to this specific number of electricaltensioners 110-117. For example, in another embodiment, an electricalriser tensioning system may include four electrical tensioners.

FIG. 1B is a block diagram illustrating a top view of a riser hybridtensioning system 100 according to one embodiment of the disclosure. Theriser 130 may be coupled to electrical tensioners 110-113 andhydro-pneumatic tensioners 120-123 by ropes. Together the electricaltensioners 110-113 and hydro-pneumatic tensioners 120-123 may form theriser hybrid tensioning system 100. Although many of the short-comingsof riser tensioning systems that employ only hydro-pneumatic risertensioners 120-123 have already been detailed, hydro-pneumatictensioners 120-123 may be used in a riser hybrid tensioning system 100to take advantage of the benefits of hydro-pneumatic tensioners 120-123.For example, a riser hybrid tensioning system 100 with hydro-pneumatictensioners 120-123 may have good reliability because the hydro-pneumatictensioners 120-123 are passive and self-contained systems that have noenergy exchange with external systems. Furthermore, the riser hybridtensioning system I 00 may be more resistant to disturbances andfluctuations of outside systems. Electrical riser tensioners 110-113 addmany advantages, such as delivering dynamically variable torque withhigh accuracy, providing quick control responses, and being easier toinstall. A riser hybrid tensioning system I 00 may therefore benefitfrom the combined advantages of hydro-pneumatic tensioning systems120-123 and electrical tensioners 110-113.

Although FIG. 1B depicts the riser hybrid tensioning system 100 withfour electrical tensioners 110-113 and four hydro-pneumatic tensioners120-123, a riser hybrid tensioning system is not limited to thisspecific number of electrical tensioners and hydro-pneumatic tensioners.For example, in another embodiment, a riser hybrid tensioning system mayinclude six hydro-pneumatic tensioners and four electrical tensioners.

FIG. 2A is block diagram illustrating a riser tensioning system 200according to one embodiment of the disclosure. The tensioning system 200may be used to control the tension of wires 231 coupling electricaltensioners 210 to a drilling riser 230. Although only one electricaltensioner 210 is illustrated, additional electrical tensioners may bepresent, such as illustrated in FIG. 1A above.

The electrical tensioner 210 may be coupled to a common DC powerdistribution bus 270, which may be shared with other electricaltensioners. The DC bus 270 provides a physical link for the energyflowing into and out of the tensioning system 200, as well as for otherpower devices. The DC bus 270 may be coupled to an active front end(AFE) rectifier 260 that converts power from an AC bus 272 powered byone or more generators 274. The power module of the AFE rectifier 260may be controlled by a power management system 250 through an AFEcontroller 260 a.

The electrical tensioner 210 may include a variable frequency drive(VFD) 211 to invert energy from AC to DC or from DC to AC. The VFD-typeinverter 211 may be controlled by the tension controller 202 through aVFD controller 211 a. In one direction, the inverter 211 may convert DCenergy from the DC bus 270 to AC energy for use by the electricaltensioner 210. In another direction, the inverter 211 may convert ACenergy from the electrical tensioner 210 to DC energy that istransferred onto the DC bus 270.

The electrical tensioner 210 may also include a motor 212 coupled by thewire 231 to a sheave 214 and to the riser 230. The motor 212 may be, forexample, a high-torque low-speed machine. The motor 212 may be adirect-drive motor, such as an axial-flux permanent magnet disc motor.The motor 212 may controlled by the VFD 211. A position sensor (PS) 216may be coupled to the electrical tensioner 210 to measure the motorrotating position 231 and to report the position to a tension controller202. A temperature sensor 218 may be located inside or on the motor 218and provide feedback to a VFD controller 211 a. For example, when atemperature measured by the sensor 218 exceeds a safe level, thecirculation of an auxiliary cooling system may be increased, or themotor 212 may be shut down to reduce its temperature.

In an all-electric tensioning system, such as illustrated in FIG. 1A,multiple electric tensioners may be coupled to the riser 230 by wires231. When the tensioning system 200 is a hybrid system, such asillustrated in FIG. 1B, the system 200 may include a hydro-pneumatictensioner 252 with associated controller 252 a. Although only onehydro-pneumatic tensioner 252 is illustrated, multiple hydro-pneumatictensioners may be coupled to the riser 230 through the wires 231. Thecontroller 252 a may also be in communication with the tensioncontroller 202.

The tension controller 202 may be configured to perform many taskswithin a hybrid or electrical riser tensioning system and providefeedback to the power management controller 250. For example, thecontroller 202 may regulate the torque in the motor 212 for differentcontrol purposes through different control algorithms. As anotherexample, the controller 202 may be used as a load sharing controllerthat distributes tension between the hydro-pneumatic tensioner 252 andthe electrical tensioner 210. Furthermore, the controller 202 may beconfigured to dynamically control the wireline 231 tension. Formonitoring and control purposes, status feedback of the electricaltensioners 210, the hydro-pneumatic tensioners 252, the riser 230 andthe drilling vessel on which the riser tensioning system is employed maybe sent to the controller 202. Alternatively, the controller 202 maycalculate the reference signals for both electrical and thehydro-pneumatic tensioners using different control algorithms. Thealgorithms may be based, in part, on the riser top and the drillingvessel heave relative positions to the seabed, velocity and accelerationfrom the motion reference unit (MRU) 232, a MRU on the vessel (notshown), and tension measurements of the electrical tensioner 210 and thehydro-pneumatic tensioner 252. Moreover, the controller 202 may beconfigured to monitor the routing of energy in and out of the electricaltensioner 202 and send this energy signal into the power managementcontroller 250.

The power management controller 250 may be configured to monitor the DCbus 270 voltage and the AC bus 272 frequency. Furthermore, thecontroller 250 may coordinate power among other power components, suchas the electrical tensioner 210, the ultra-capacitor bank 222, and thepower dissipater 242.

Referring back to FIG. 2A, in normal operation, a drilling vessel havinga riser hybrid tensioning system may experience wave motion thattransfers large amounts power to and/or from the electrical tensioner210. For example, when the vessel experiences waves that cause thevessel to move downward, the electrical tensioner 210 may consume energyfrom the rig power network 250. The energy consumed by the electricaltensioner 210 may be in the megajoule range, and the required peak powermay then be in the megawatt range. When the vessel experiences wavesthat cause the vessel to move upward, the electrical tensioner 210 mayrelease the same power back onto the DC bus 270. Power fluctuations fromthe waves may be compensated with elements 222 and 242. That is, bystoring energy returned to the DC bus 270 by the energy storage elements222 or dissipating the energy in energy dissipation elements 242.

The energy storage elements 220 may be coupled to the DC bus 270. Eachenergy storage element 222 may be coupled to a DC/DC power chopper(DDPC) 221. The specific number and type of energy storage devices 222used for the energy storage elements 220 may depend on applicationspecific parameters, such as the type of vessel used or the spaceavailable for the energy storage elements 220. An energy storage device222 may be, for example, an ultracapacitor bank (UCB) a battery bank, ora flywheel. When the UCB is used for the energy storage device 222, theUCB may be selected to have a capacity at least 1.2 times the maximum ofboth the vessel heave of the most significant sea state criterion andfive times of the UCB's capacity de-rating.

The tensioning system 200 may also include a power dissipater 242coupled to the DC bus 270 through a unidirectional power chopper 241.The unidirectional power chopper 241 which may regulate the amount ofenergy to be dissipated by the power dissipater 242. The powerdissipater 242 may be any device that consumes energy, such as aresistor or a heat sink. Operation algorithms within the powermanagement system 250 may route energy into power dissipaters 242 whenthe energy storage devices 222 are fully charged or when the operatingvoltages of the UCBs exceed a maximum operating voltage.

FIG. 3A shows a flow chart illustrating a method 300 for controlling thetension of a riser tensioning system according to one embodiment of thedisclosure. The method 300 begins at block 302 with measuring a tensiondelivered by a tensioner within the riser tensioning system. Themeasured tension may be the tension delivered by a hydro-pneumatictensioner or an electrical tensioner. In one embodiment, a controller,such as the controller 202 of FIG. 2A, may receive tension feedbacksignals delivered by the hydro-pneumatic or electrical tensioner toobtain the measured tension delivered by either the hydro-pneumatic orelectrical tensioner. In certain embodiments, a plurality ofhydro-pneumatic and/or electrical tensioners may be monitored by thecontroller. In one embodiment, a controller, such as the controller 202of FIG. 2A, may measure the tension delivered by the hydro-pneumatic orelectrical tensioners, while in tensioner.

At block 304, a desired tension for a plurality of electrical tensionersmay be determined based, in part, on the measured tension at block 302.Other parameters that may be used to determine the desired tension for aplurality of electrical tensioners include the tension delivered by ahydro-pneumatic or electrical tensioner, a total required tension of theentire riser tensioning system, a total number of hydro-pneumatictensioners in a riser hybrid tensioning system, and/or a total number ofelectrical tensioners in the system. Furthermore, the controller 202 ofFIG. 2A may be configured to determine the desired tension of theelectrical tensioner based, in part, on monitored parameters of adrilling vessel, such as the total number of hydro-pneumatic andelectrical tensioners on the vessel.

At block 306, the desired tension of block 304 may be distributed to theplurality of electrical tensioners. The plurality of electricaltensioners may then be controlled to deliver the determined tension byevenly rolling in or rolling out a wire coupled to a respectiveelectrical tensioner of the plurality of electrical tensioners.

According to one embodiment, the desired tension of an electricaltensioner, or a plurality of electrical tensioners, may be calculatedusing the following equation:

${{T_{ETi}(t)} = {\left( {{T_{Total}(t)} - {\sum\limits_{i = 1}^{n_{HT}}{T_{HTi}(t)}}} \right)/n_{ET}}},$

where T_(ETi) may denote the desired tension of an individual electricaltensioner i, and T_(HTi) may be the tension delivered by hydro-pneumatictensioner i at any given time, and T_(Total) may represent the totaldesired tension of the entire riser hybrid tensioning system. The n_(HT)and n_(ET) parameters may be the total number of hydro-pneumatic andelectrical tensioners, respectively, in the system.

At block 308, the plurality of tensioners may be controlled based, inpart, on the tension that was determined at block 304 and that wasdistributed at block 306. For example, the tensioners may apply atension to the wires. The plurality of electrical tensioners may becontrolled and coordinated to satisfy different control purposes. Thismay assist in stabilizing a riser in an offshore drilling vessel. Forexample, the measuring of the tension delivered by tensioners may beperformed continuously to dynamically calculate the desired tension of atensioner and control the tension being delivered by tensioners. Thismay ensure that the total delivered tension by the hydro-pneumaticand/or electrical tensioners remains nearly constant. In one embodiment,the controller 202 of FIG. 2A may be configured to control the pluralityof electrical tensioners and adjust the wireline tension according todifferent drilling operation and sea condition. The actions disclosed atthe blocks of FIG. 3A may be performed continuously, and in parallel,with the actions that manage the energy in the system, such as thosedescribed at blocks 330 and 340 of FIG. 3B.

FIG. 3B is a flow chart illustrating a method for controlling energytransfer within a riser tensioning system according to one embodiment ofthe disclosure. The actions of method 300 of FIG. 3A may be performedcontinuously, and either sequentially or in parallel, with the actionsof method 350 of FIG. 3B.

At block 320, it is determined whether a vessel has moved vertically upor down. In one embodiment, the vessel being monitored for verticalmovement may be an offshore drilling vessel on which a riser tensioningsystem, as in FIG. 1A, or riser hybrid tensioning system, as in FIG. 1B,is located. The vertical motion of the vessel may be caused by waves inthe ocean.

At block 320, when the vessel has moved down, the method 350 may proceedto block 330 where energy may be transferred from an electricaltensioner to energy storage devices. That is, the motor of theelectrical tensioning system may act as a generator when the vesselmovesup. At block 330, the energy from an electrical tensioner may betransferred to the energy storage system or to power dissipaters fordissipating the energy generated by the electrical tensioner. The energytransferred from an electrical tensioner may be energy that has beengenerated by the electrical tensioner. For example, when the vesselmoves up, the wire coupled to the electrical tensioner may roll out. Asthe wire rolls out, the motors may act as generators convertingpotential energy to AC electrical energy. The generated AC electricalenergy may be inverted to DC energy by an AC/DC inverter and flow onto acommon DC power distribution bus where it may then be transferred to theenergy storage devices for storage.

Decisions may be made to determine where the energy generated from anelectrical tensioner should be routed. For example, at block 331, it isdetermined if an energy storage device has reached its maximum energycapacity. At block 332, the energy generated by an electrical tensionermay be transferred to the energy storage device for storage if it wasdetermined at block 331 that the energy storage device had not reachedits maximum capacity. Energy generated by an electrical tensioner maycontinue to be stored in the energy storage device or devices until theenergy storage device or devices have reached their maximum energycapacity. As energy is stored in the energy storage device or devices,the energy in the energy storage device or devices may be monitored todetermine at block 331 if the maximum energy capacity has been reached.

After the determination at block 331 that the energy storage devices inthe electrical tensioning system have reached their maximum energycapacity, it may be determined at block 333 if a power network hasreached capacity. In an embodiment, a safe operation criterion orthreshold for the power network may serve as an aid in determiningwhether the power network has reached capacity. At block 334, the energygenerated by an electrical tensioner may be transferred to the AC powernetwork for other power consumption if it was determined at block 333that the power network had not reached its maximum capacity. Energygenerated by an electrical tensioner may continue to be transferred intothe AC power network until the power network has reached its maximumenergy capacity. As energy is absorbed in the power network, thefrequency of the power network may be monitored to determine at block333 if the maximum energy capacity has been reached. At block 336, theenergy generated by an electrical tensioner may be transferred to apower dissipating device to dissipate excess generated energy if it wasdetermined at block 333 that the power network had reached its maximumcapacity.

If it is determined at block 320 that the vessel has moved down, themethod 350 may proceed to block 340 where energy may be transferred fromenergy storage devices to the electrical tensioner. For example, whenthe vessel moves down, the wire coupled to the electrical tensioner mayroll in. Energy stored in energy storage devices may be transferred ontothe common DC power distribution bus where it can be transferred to anelectrical tensioner. The energy transferred from the energy storagedevices to the DC bus may be inverted to AC energy by the AC/DC inverterin an electrical tensioner. The inverted AC energy may be converted fromAC electrical energy to potential energy by the motor in an electricaltensioner to control the tension in the wire. The energy stored in theenergy storage device that is transferred to an electrical tensioner maybe energy that has been stored in the energy storage device when thevessel last moved down or energy that was provided by charging from thepower network.

At block 340, the energy transferred to the electrical tensioner mayalso be transferred from the AC power network. Furthermore, energy froma power network may also be transferred to an energy storage device tocharge it at block 340.

Decisions may be made to determine from where energy for an electricaltensioner should be routed. For example, at block 341, it is determinedif an energy storage device has sufficient energy stored. In anembodiment, an energy storage device that has sufficient energy storedmay be one that has energy amounting to a predetermined percentage ofits maximum capacity. For example, a minimum level in a UCB may be 20%of a total capacity or 40% of a nominal voltage. At block 342, energymay be transferred to an electrical tensioner from an energy storagedevice if it was determined at block 341 that the energy storage devicehad sufficient energy stored. Furthermore, at block 342, the energytransferred to an electrical tensioner may be transferred from aplurality of energy storage devices if it was determined at block 331that the plurality energy storage devices had sufficient energy, and theenergy transferred may be transferred to a plurality of electricaltensioners. Energy may continue to be transferred to an electricaltensioner from the energy storage device or devices until the energystorage device or devices have become depleted or become dischargedbelow a predetermined percentage of the maximum capacity. As energy istransferred from the energy storage devices, the energy in the energystorage devices may be monitored to determine at block 341 if they havesufficient energy to continue operating the electric tensioners.

According to an embodiment, after the determination at block 341 thatthe energy storage devices in the electrical tensioning system do nothave sufficient energy, at block 344, the energy transferred to anelectrical tensioner may be transferred from the DC bus. For example,additional power may be transferred from generators to the DC busthrough an AC-to-DC converter. Furthermore, energy may be transferredfrom the DC bus to the energy storage devices that are discharged ordepleted to charge the energy storage devices. By charging the depletedenergy storage devices, the energy required by electrical tensioners maybe transferred from the energy storage devices the next cycle the vesselmoves up.

Through the management of energy described in method 350 of FIG. 3B, theelectrical tensioning system may be an independent energy conversionsystem with nearly zero energy consumption from the DC bus other thanlosses by the tensioners.

FIG. 4A is a graph illustrating a relationship between vessel positionand riser tension according to one embodiment of the disclosure. Thevessel position versus time graph 402 provides an illustration of themovement that a vessel may experience. When the vessel moves down, suchas during a region 430, an electrical tensioner may receive energy fromeither the energy storage devices or the power network. In oneembodiment, during the time region 430, the actions at block 340 of FIG.3B may be performed, because the decision at block 320 may determinethat the vessel moved vertically down during this time region. When thevessel moves up, such as during a region 440, an electrical tensionermay generate energy that can be stored in the energy storage system,transferred to the power network, or dissipated in a power dissipater.Furthermore, the actions at block 330 of FIG. 3B may be performed,because the decision at block 320 may determine that the vessel moved upduring this time region.

The riser tension versus time graph 404 provides an illustration of thetotal tension delivered by the hydro-pneumatic and/or electricaltensioners across time. The total tension 410 may be maintained nearlyconstant at all times despite the vessel's vertical positionfluctuations indicated in the vessel position versus time graph 402.

FIG. 4B is a graph illustrating a relationship between vessel velocityand riser tension according to one embodiment of the disclosure. A graph452 traces vertical velocity of a vessel experiencing waves in an ocean.A graph 454 traces tension delivered to a wire during the same timeperiod as graph 452. During a first half of the wave period while thevessel is falling, a smaller tension is applied to the line in timeperiod 464. During time period 464, less energy is converted topotential energy by the electric tensioners. During the second half ofthe wave period while the vessel is rising, a larger tension is appliedto the line in time period 462. During time period 462, electricalenergy may be harvested from the wave motion in order to compensate thesystem losses and to increase the reliability during AC power networkblack out situation.

The overall performance of a riser hybrid tensioning system isillustrated in FIG. 4C, which illustrates graphs of tensions within theriser hybrid tensioning system according to one embodiment. FIGS. 4A-4Cillustrate the AC portion of the tensions. The y-axis of each graphignores the DC portion of the tensions. Each of the tensions may benearly constant, only varying in a small range as shown in the ACportions. A graph 464 illustrates a required load tension as measured atthe top of a riser. A graph 464 illustrates tension delivered by ahydro-pneumatic tensioner, and a graph 466 illustrates tension deliveredby an electric tensioner. The tension applied by the electric tensionerin graph 466 is 180 degrees out of phase from the tension applied by thehydro-pneumatic tensioner in graph 464, such that the summation of thetension delivered by the hydro-pneumatic tensioner and the electrictensioner provides the required tension illustrated in graph 462. Inusing the riser hybrid tensioning disclosed above, heave compensation,which may be controlled by the controller 202 of FIG. 2A, may have ahigher level of accuracy. Thus, the riser cyclical fatigue life may beimproved by using the riser hybrid tensioning system.

FIG. 5 is an illustration 500 of the routing of energy in a riser hybridtensioning system according to one embodiment of the disclosure. Theillustration 500 may visually depict the management and routing ofenergy as described in FIG. 3B. In one embodiment, the AC power network550, power dissipater 540, tensioner 510, and the ultra-capacitor bank520 in FIG. 5 may be the AC power network 272, power dissipater 240,electrical tensioner 210, and the energy storage device 220 described inFIG. 2A, respectively. As one example, arrow 502 illustrates that energymay be transferred from a UCB 520 to an electrical tensioner 510 asdescribed at block 342 of FIG. 3C. In one embodiment, the controlling ofthe routing of energy to and from different elements within the riserhybrid tensioning system may be performed by the controller 250 of FIG.2A.

FIG. 6 depicts a control scheme 600 for energy storage devices accordingto one embodiment of the disclosure. In this embodiment an energystorage device to be controlled may be a ultra-capacitor bank (UCB), andthe DC/DC power chopper DDPC 620 in FIG. 6 may be the DDPC 221 of FIG.2A. According to the embodiment, a feedback controller with fastersampling rate may be used to regulate the power, voltage, and currentinside of each UCB based on a signal received from the power managementcontroller. An outer power control loop may defines a UCB voltage setpoint. a control loop, which may predefine a UCB voltage set point, mayforce a UCB to supply or absorb power according to a kW referencereceived from an upper-level coordination controller, such as thecontroller 250 of FIG. 2A. A difference 623 between a reference power621 and a measured UCB power 622 may be transmitted through a powerregulator 624 that may set an UCB voltage reference 602. A difference606 between a reference voltage 602 and a measured UCB voltage 604 maybe transmitted through a voltage regulator 608 that may set an UCBcurrent reference 610. Furthermore, the DDPC's duty cycle 618 may begenerated by a current regulator 616 based on an error 614 between thecurrent reference 610 and a measured current 612. This control scheme600 may enable UCBs to compensate for energy demand in a tensionersystem. The control scheme may be implemented with a controller 630,which may control more than one DDPC 620 in parallel.

A power management controller may be used in this topology to keepenergy equalized in each UCB, in order to avoid over-depletion of acertain UCB, so that the life cycles of all UCBs are balanced. When anenergy surge is regenerated from the electrical tensioners, the amountof power flowing into an energy storage system may be distributed toeach UCB according to the percentage of its free volume versus the totalfree volume of all UCBs, as shown in

$P_{i} = {\frac{C_{i}\left( {V_{i\; \_ \; {FULL}}^{2} - V_{i}^{2}} \right)}{\begin{matrix}{{C_{1}\left( {V_{1\; \_ \; {FULL}}^{2} - V_{1}^{2}} \right)} + K +} \\{{C_{i}\left( {V_{i\; \_ \; {FULL}}^{2} - V_{i}^{2}} \right)} + K + {C_{n}\left( {V_{n\; \_ \; {FULL}}^{2} - V_{n}^{2}} \right)}}\end{matrix}}P_{TOTAL}}$

where P_(i) with u=1, . . . , n is the power distributed to the i^(th)UCB, P_(TOTAL) is the total power regenerated from the tensioningsystem, C_(i) is the capacitance of the i^(th) UCB, V_(i) and V_(i_FULL)are the actual voltage and the nominal voltage of the i^(th) UCB.When energy is consumed by electrical tensioners, the amount of thepower transferred out of the energy storage system may be withdrawn fromeach UCB according to the percentage of its state of charge (SOC) versusthe total SOC of all UCBs, as shown in:

$P_{i} = {\frac{C_{i}V_{i}^{2}}{{C_{1}V_{1}^{2}} + K + {C_{i}V_{i}^{2}} + K + {C_{n}V_{n}^{2}}}P_{TOTAL}}$

With the novel riser hybrid tensioning system disclosed, several controlmodes employed in riser control systems may be enhanced, such as activeheave compensation control, anti-recoil control, vortex-inducedvibration (VIV) suppression control, and riser position control. Quickerresponse times provide a dynamic response profile that may prevent theriser from jumping out during anti-recoil operation. Furthermore, theriser hybrid tensioning system may deliver variable tensions that mayactively suppress VIV.

Several control modes may be implemented that utilize the riser hybridtensioning system disclosed above, such as an active heave compensationcontrol mode. In this control mode the electrical tensioning system maybe set to track a desired vessel heave trajectory in the riser topreference frame to keep the tension applied at the riser top to bewithin a safe range.

The entire active heave compensation control algorithm may be embeddedinto the controller 202 in FIG. 2A to calculate torque references and tocontrol the active heave compensation system. The calculated referencesignals can be input into an AC/DC inverter to effectively control themotor to roll in or roll out the wire in the electrical tensioningsystem so as to optimize the total delivered tension by both electricaland hydro-pneumatic tensioners for compensating the force disturbancesinduced on riser and the acceleration of all moving mechanics, as shownin FIG. 4C. In using the riser hybrid tensioning system disclosed above,heave compensation, which may be controlled by the controller 202 ofFIG. 2A, may have an improved control response time and a higher levelof accuracy. Thus, the riser cyclical fatigue life may be improved byusing the riser hybrid tensioning system.

In one embodiment, another control mode that may be used is ananti-recoil mode to bring the riser string up in a controlled manneraccording to a desired goal such as to achieve a desired water clearancefrom the riser bottom to the top of LMRP or to maintain a safe air gapdistance from the drill floor to the riser top at the instant of endstop. In this control mode, the control strategy for the electricaltensioner may be a fixed relationship function between the motor outputtorque and the wire relevant displacement. The fixed relationshipstrategy may be embedded into a controller, such as the controller 202of FIG. 2A, to control the electrical tensioners during an emergencydisconnect scenario in which the riser tensioning system may be in ananti-recoil mode. Another embodiment for anti-recoil control using theriser hybrid tensioning system may include a feedback control strategythat controls the tension delivered by electrical tensioners and itsrelative displacement to achieve a controlled deceleration profile ofthe riser string until it stops. This control algorithm for theanti-recoil mode may also be embedded into a controller. For example,the controller 202 of FIG. 2A, when operating in anti-recoil mode, maybe configured to control the electrical tensioners to reduce the upperpulling force on a drilling riser.

FIG. 2B is a block diagram illustrating an anti-recoil controller forthe riser tensioning system according to one embodiment of thedisclosure. A controller 290 may include cascadeproportional-integral-derivative (PID) controllers for controlling ariser hybrid tensioning system. A first PID controller 292 may receive areference position signal POS from the controller 202 of FIG. 2A, and afeedback signal (FB) from an electric tensioner (ET) drive 296 from theposition sensor 216 of FIG. 2A. The first PID controller 292 may be anouter loop of the controller 290 for performing wire-line displacementcontrol. The output of the first PID controller 292 is provided as aninput to a second PID controller 294, which also receives informationregarding the vessel velocity (V), such as from the motion referenceunit (MRU) 233 sitting on the vessel body of FIG. 2A, and a feedbacksignal (FB2) from the ET drive 296. The second PID controller 294 may bean inner loop of the controller 290 for performing wire-line velocitycontrol.

An anti-recoil trigging method may be comparing the relative verticalmovement between the MRU 232 of FIG. 2A located on the riser and an MRU233 of FIG. 2A on the vessel body. If the difference exceeds a certainlimit, the anti-recoil system may be triggered.

Furthermore, a riser-mounted MRU may measure second-order transientshock waves in the riser caused by riser disconnection. Because thesecond-order transient shock wave travels along the riser at a muchfaster rate than velocity of the riser main body, recoil of the risermay be detected quicker by monitoring the second-order transient shockwave. When a shock wave is detected, hydro-pneumatic tensioners may beunloaded from the riser and the electrical tensioners could adjusttension on the riser to counteract the riser recoil.

The riser hybrid tensioning system may operate in a control mode for VIVsuppression that compensates the disturbances induced at the top of ariser to reduce the VIV and extend riser fatigue life. A comparison ofrelative horizontal position or velocity may be performed between theMRU 232 of FIG. 2A located on the riser and an MRU 233 of FIG. 2A on thevessel body. With a suitable model for the riser and a suitable controlalgorithm, the electrical tensioner controlled by the controller 202 ofFIG. 2A may decrease the VIV magnitude and frequency, therefore reducethe fatigue damage of the riser pipe and increase the whole risersystems availability. Using riser hybrid tensioning system could be setto stabilize the riser top at the small neighborhood of its originalposition, i.e., to reduce the vibration displacement of the riser in xand y axis in transverse reference plane. The destructive vortex-inducedvibration is in fact an unsteady resonant oscillation condition thatcauses the riser fatigue failure over time. Another VIV control strategymay set to prevent the riser string vortex shedding from entering theriser natural frequency by applying dynamic top tensions in verticaldirections. For example, the VIV pattern in water may be collapsed byintroducing a small disturbance into the resonant potential and kineticenergy from the top of the riser.

An active riser position control may be applied using this hybrid risertensioning system, implemented in the controller 202 of FIG. 2A toposition and/or relocate a riser string. For example, a riser stringdisconnected from a blow-out preventer (BOP) may hang from the vesselwhile the vessel relocates to a new well center. During this time, theriser string may act as a spring that amplifies waves in the ocean.Electrical tensioners may be used to control the accurate position inwater to eliminate the mass spring effect in the riser string duringmovement of the riser string from one well center to another wellcenter.

Electric tensioners may also be used to reconnect a lower marine riserpackage (LMRP) at the end of a riser string back onto blowout preventer.The riser hybrid tensioning system may provide precise LMRP positioncontrol which may reduce the time consumed in reconnecting the LMRP ontoa blowout preventer (BOP) in comparison a hydro-pneumatic system. Theriser hybrid tensioning system may directly and securely land the LMRPback onto the BOP through the leveraging of the electrical tensionerswith proper maneuver of remotely operated vehicles. Furthermore, anoperator may control the appropriate distance between the LMRP and theBOP. The controller, now operating in riser reconnection mode, may beconfigured and operated in position control mode to control the distancebetween the LMRP and the BOP by compensating vessel heave motion.According to one embodiment, the LMRP may be coupled to the BOP, suchthat the LMRP and BOP are being placed on a well head together throughthe position control by the hybrid tensioners.

Electric tensioners may also facilitate movement of a riser string froma first drilling station to another drilling station on a dual-activityvessel. For example, a first drilling station may construct the wellhead, and a second station may construct the riser string. Then, theelectric tensioners may adjust lengths of wire coupled to the riserstring to move the riser string from the second drilling station to thefirst drilling station. FIGS. 7A and 7B are block diagrams illustratingmovement of a riser string between drilling stations by electrictensioners according to one embodiment of the disclosure. FIG. 7Aillustrates a riser string 702 attached to a derrick 710. The riserstring 702 may be held in place by electric tensioners 730 and 732. Whenthe riser string 702 is attached to a second drilling station, wirescoupling the electric tensioner 732 may be at high tension to roll thesheaves 722 towards the first station and also reduce length of thewires and, thus, the distance between the tensioner 732 and the riserstring 702. FIG. 7B illustrates the riser string 702 attached to aderrick 710 above a first drilling station. Wires coupling the electrictensioner 730 may be adjusted to roll the sheaves 722 towards the secondstation and to reduce length of the wires and, thus, the distancebetween the riser string 702 and the tensioner 730. The tensioners 730and 732 may be coupled to the riser 702 through sheaves 722 attached toa rack 720 on the vessel.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thepresent processes, disclosure, machines, manufacture, compositions ofmatter, means, methods, or steps, presently existing or later to bedeveloped that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present disclosure. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

What is claimed is:
 1. An apparatus, comprising: a drilling riser; a vessel; a first motion reference unit (MRU) coupled to the drilling riser, configured to measure one or more properties of the drilling riser; a second MRU coupled to the vessel, configured to measure one or more properties of the vessel; a plurality of wires coupled to the drilling riser; a first and second electrical tensioner coupled to the drilling riser via a first and a second wire of the plurality of wires; and a controller configured to perform steps comprising: distributing tension to the first and second electrical tensioners; receiving the one or more properties of the drilling riser from the first MRU; receiving the one or more properties of the vessel from the second MRU; determining a tension based, at least in part, on the one or more properties of the drilling riser and the one or more properties of the vessel; and controlling the first and second electrical tensioners to adjust a tension of the first and second wires based, at least in part, on the determined tension.
 2. The apparatus of claim 1, wherein the controller comprises: a first proportional-integral-derivative (PID) controller; and a second PID controller, wherein determining the tension based, at least in part, on the one or more properties of the drilling riser and the one or more properties of the vessel comprises performing wire-line displacement control by the first PID controller and performing wire-line velocity control by the second PID controller.
 3. The apparatus of claim 2, wherein an output of the first PID controller is provided as an input to the second PID controller for performing wire-line velocity control.
 4. The apparatus of claim 1, wherein the one or more properties of the drilling riser comprise a vertical movement of the drilling riser, wherein the one or more properties of the vessel comprise a vertical movement of the vessel, wherein the controller is further configured to perform steps comprising: determining a difference between the vertical movement of the drilling riser and the vertical movement of the vessel; detecting the difference exceeds a predetermined threshold; and entering an anti-recoil mode when a difference exceeding the predetermined threshold is detected.
 5. The apparatus of claim 1, wherein the one or more properties of the riser comprise a horizontal position of the riser, wherein the one or more properties of the vessel comprise a horizontal position of the vessel, wherein the step of determining comprises determining the tension based, at least in part, on a comparison of the horizontal position of the riser with the horizontal position of the vessel, and wherein the step of controlling comprises controlling the first and second electrical tensioners, in a vortex-inducted vibration (VIV) mode, to adjust the tension of the first and second wires, based, at least in part, on the determined tension to reduce VIV of the riser.
 6. The apparatus of claim 4, wherein controlling the first and second electrical tensioners in the VIV mode comprises controlling the first and second electrical tensioners to stabilize a top of the drilling riser within a predetermined range of an original position of the top of the drilling riser.
 7. The apparatus of claim 4, wherein controlling the first and second electrical tensioners in the VIV mode comprises controlling the first and second electrical tensioners to apply dynamic tensions to the first and second wires in one or more vertical directions to prevent a VIV frequency of the drilling riser from entering a natural frequency range of the drilling riser.
 8. The apparatus of claim 1, wherein the controller is further configured to perform steps comprising: determining a relative position of a top of the drilling riser to a seabed based, at least in part, on the received one or more properties of the drilling riser; determining a relative position of the vessel to the seabed based, at least in part, on the received one or more properties of the vessel, wherein the step of determining a tension comprises determining a tension based, at least in part, on the determined relative position of the top of the drilling riser and the determined relative position of the vessel.
 9. The apparatus of claim 1, further comprising: a hydro-pneumatic tensioner coupled to the drilling riser via a third wire of the plurality of wires, wherein the controller is further configured to distribute tension to the hydro-pneumatic tensioner, and wherein the controller is further configured to control the hydro-pneumatic tensioner to adjust a tension of the third wire based, at least in part, on the determined tension.
 10. The apparatus of claim 1, wherein the controller is further configured to perform steps comprising: detecting second-order transient shockwaves in the received one or more properties of the drilling riser indicating a disconnect between the drilling riser and a blow-out preventer (BOP), wherein the step of determining comprises determining the tension based, at least in part, on the detected second-order transient shockwaves, and wherein the step of controlling comprises controlling the first and second electrical tensioners to adjust the tension of the first and second wires based, at least in part on the determined tension to counter riser recoil resulting from the disconnect between the drilling riser and the BOP.
 11. A method, comprising: receiving one or more properties of a drilling riser from a first motion reference unit (MRU) coupled to the drilling riser; receiving one or more properties of a vessel from a second MRU coupled to the vessel; determining tensions for each of a plurality of electrical tensioners based, at least in part, on the received one or more properties of the drilling riser and the received one or more properties of the vessel; controlling the plurality of electrical tensioners based, at least in part, on the determined tensions.
 12. The method of claim 11, wherein the step of determining tensions comprises: performing wire-line displacement control by a first PID controller; and performing wire-line velocity control by a second PID controller.
 13. The method of claim 11, wherein the step of determining tensions further comprises providing an output of the first PID controller as an input to the second PID controller for performing wire-line velocity control.
 14. The method of claim 12, wherein the one or more properties of the drilling riser comprise a vertical movement of the drilling riser, wherein the one or more properties of the vessel comprise a vertical movement of the vessel, the method further comprising: determining a difference between the vertical movement of the drilling riser and the vertical movement of the vessel; detecting the difference exceeds a predetermined threshold; and entering a controller of the plurality of electrical tensioners into an anti-recoil mode when a difference exceeding the predetermined threshold is detected.
 15. The method of claim 12, wherein the one or more properties of the riser comprise a horizontal position of the riser, wherein the one or more properties of the vessel comprise a horizontal position of the vessel, wherein the step of determining comprises determining the tension based, at least in part, on a comparison of the horizontal position of the riser with the horizontal position of the vessel, and wherein the step of controlling comprises controlling the plurality of electrical tensioners, in a vortex-inducted vibration (VIV) mode, to adjust the tension of the first and second wires, based, at least in part, on the determined tension to reduce VIV of the riser.
 16. The method of claim 14, wherein controlling the plurality of electrical tensioners in the VIV mode comprises controlling the plurality of electrical tensioners to stabilize a top of the drilling riser within a predetermined range of an original position of the riser.
 17. The method of claim 14, wherein controlling the plurality of electrical tensioners in the VIV mode comprises controlling the plurality electrical tensioners to apply dynamic tensions to the first and second wires in one or more vertical directions to prevent a VIV frequency of the riser from entering a natural frequency range of the riser.
 18. The method of claim 12, wherein the step of determining comprises determining a tension for a plurality of electrical tensioners based, at least in part, on the received one or more properties of the drilling riser and the received one or more properties of the vessel.
 19. The method of claim 17, further comprising: determining a relative position of a top of the drilling riser to a seabed based, at least in part, on the received one or more properties of the drilling riser; determining a relative position of the vessel to the seabed based, at least in part, on the received one or more properties of the vessel, wherein the step of determining the tension comprises determining the tension based, at least in part, on the determined relative position of the top of the drilling riser and the determined relative position of the vessel.
 20. The method of claim 11, further comprising: distributing tension to a hydro-pneumatic tensioner; and controlling the hydro-pneumatic tensioner based, at least in part, on the determined tension. 